In a variable geometry turbocharger (VGT), the supercharging efficiency is improved by changing the area of an exhaust flow passage in an exhaust turbine in accordance with the rotation speed of the engine to control the exhaust flow rate to the exhaust turbine blade. Such a control on the area of an exhaust flow passage is performed by driving a nozzle mechanism with an actuator. For instance, if the engine rotation speed is relatively low, the area of the exhaust flow passage is narrowed (reduced) with the nozzle mechanism to increase the flow rate of exhaust gas and increase the output torque. If the engine rotation speed is relatively high, the area of the exhaust flow passage is expanded (increased) by opening (increasing) the nozzle mechanism to improve the fuel consumption.
Meanwhile, if a trouble like sticking occurs in a nozzle mechanism, which is an important constituent element of a variable geometry turbocharger, it is no longer possible to adjust (narrow or expand) the area of an exhaust flow passage as described above. To prevent such troubles, it is important to detect at an early stage an abnormality such as a trouble in a nozzle mechanism and a behavior that seems to develop a trouble.
As a technique related to abnormality detection of such a type of variable geometry turbocharger, Patent Document 1 discloses, for instance, performing a wiping motion that drives a nozzle mechanism from a full-open position to a full-closed position when the engine is stopped, and detecting an abnormality on the basis of the time required for the wiping motion.